Structure and Method for Gate-All-Around Metal-Oxide-Semiconductor Devices With Improved Channel Configurations

ABSTRACT

The present disclosure provides an integrated circuit device comprising a semiconductor substrate having a top surface; a first source/drain feature and a second source/drain feature over the semiconductor substrate; semiconductor layers connecting the first source/drain feature and the second source/drain feature, the semiconductor layers stacked over each other along a first direction normal to the top surface; each of the semiconductor layers having a center portion of a first thickness and two end portions of a second thickness larger than the first thickness; each of the two end portions connecting the center portion and one of the first and the second source/drain features; a gate electrode engaging the center portion of each of the semiconductor layers; a first spacer over the two end portions of a topmost semiconductor layer of the semiconductor layers; and a second spacer between vertically adjacent end portions of the semiconductor layers.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing, and for these advancements to berealized, similar developments in IC processing and manufacturing areneeded.

For example, multi-gate devices have been introduced in an effort toimprove gate control by increasing gate-channel coupling, reduceOFF-state current, and reduce short-channel effects (SCEs). One suchmulti-gate device is gate-all-around (GAA) transistor, whose gatestructure extends around its channel region providing access to thechannel region on all sides. The GAA transistors are compatible withconventional complementary metal-oxide-semiconductor (CMOS) processes,allowing them to be aggressively scaled down while maintaining gatecontrol and mitigating SCEs. However, conventional methods for GAAdevices may experience challenges including poor epitaxial growth in thesource/drain region and small formation margin for gate dielectric andelectrode in the narrow channel-channel spaces.

Therefore, although conventional GAA devices have been generallyadequate for their intended purposes, they are not satisfactory in everyrespect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A, 1B, and 1C are flow charts of an example method forfabricating an embodiment of a GAA device of the present disclosureaccording to some embodiments of the present disclosure.

FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A, 15A, 16A,17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, 25A, 26A, 27A, 28A, 29A, 30A,31A, 32A, 33A, 34A, 35A, 36A, 37A, and 38A are top views of embodimentsof GAA devices of the present disclosure constructed at variousfabrication stages according to some embodiments of the presentdisclosure.

FIGS. 2B, 3B, 4B, 5B, 6B, 7B, 8B, 9B, 10B, 11B, 12B, 13B, 14B, 15B, 16B,17B, 18B, 19B, 20B, 21B, 22B, 23B, 24B, 25B, 26B, 27B, 28B, 29B, 30B,31B, 32B, 33B, 34B, 35B, 36B, 37B, and 38B are cross sectional views ofembodiments of GAA devices of the present disclosure along the line A-A′in FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A, 15A,16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, 25A, 26A, 27A, 28A, 29A,30A, 31A, 32A, 33A, 34A, 35A, 36A, 37A, and 38A, respectively, accordingto some embodiments of the present disclosure.

FIGS. 2C, 3C, 4C, 5C, 6C, 7C, 8C, 9C, 10C, 11C, 12C, 13C, 14C, 15C, 16C,17C, 18C, 19C, 20C, 21C, 22C, 23C, 24C, 25C, 26C, 27C, 28C, 29C, 30C,31C, 32C, 33C, 34C, 35C, 36C, 37C, and 38C, are cross sectional views ofan embodiment of a GAA device of the present disclosure along the lineB-B′ in FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A,15A, 16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, 25A, 26A, 27A, 28A,29A, 30A, 31A, 32A, 33A, 34A, 35A, 36A, 37A, and 38A, respectively,according to some embodiments of the present disclosure.

FIGS. 2D, 3D, 4D, 5D, 6D, 7D, 8D, 9D, 10D, 11D, 12D, 13D, 14D, 15D, 16D,17D, 18D, 19D, 20D, 21D, 22D, 23D, 24D, 25D, 26D, 27D, 28D, 29D, 30D,31D, 32D, 33D, 34D, 35D, 36D, 37D, and 38D, are cross sectional views ofan embodiment of a GAA device of the present disclosure along the lineC-C′ in FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A,15A, 16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, 25A, 26A, 27A, 28A,29A, 30A, 31A, 32A, 33A, 34A, 35A, 36A, 37A, and 38A, respectively,according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Still further, when a number or a range of numbers is described with“about,” “approximate,” and the like, the term is intended to encompassnumbers that are within a reasonable range including the numberdescribed, such as within +/−10% of the number described or other valuesas understood by person skilled in the art. For example, the term “about5 nm” may encompass the dimension range from 4.5 nm to 5.5 nm.

The present disclosure is generally related to ICs and semiconductordevices and methods of forming the same. More particularly, the presentdisclosure is related to gate-all-around (GAA) devices. A GAA deviceincludes any device that has its gate structure, or portions thereof,formed around all-sides of a channel region (e.g. surrounding a portionof a channel region). In some instances, a GAA device may also bereferred to as a quad-gate device where the channel region has foursides and the gate structure is formed on all four sides. The channelregion of a GAA device may include one or more semiconductor layers,each of which may be in one of many different shapes, such as wire (ornanowire), sheet (or nanosheet), bar (or nano-bar), and/or othersuitable shapes. In embodiments, the channel region of a GAA device mayhave multiple horizontal semiconductor layers (such as nanowires,nanosheets, or nano-bars) (hereinafter collectively referred to as“nanochannels”) vertically spaced, making the GAA device a stackedhorizontal GAA device. The GAA devices presented herein may be acomplementary metal-oxide-semiconductor (CMOS) GAA device, a p-typemetal-oxide-semiconductor (pMOS) GAA device, or an n-typemetal-oxide-semiconductor (nMOS) GAA device. Further, the GAA devicesmay have one or more channel regions associated with a single,contiguous gate structure, or multiple gate structures. One of ordinaryskill may recognize other examples of semiconductor devices that maybenefit from aspects of the present disclosure. For example, other typesof metal-oxide semiconductor field effect transistors (MOSFETs), such asplanar MOSFETs, FinFETs, other multi-gate FETs may benefit from thepresent disclosure.

In the illustrated embodiments, the IC device includes a GAA device 100.The GAA device 100 may be fabricated during processing of the IC, or aportion thereof, that may comprise static random access memory (SRAM)and/or logic circuits, passive components such as resistors, capacitors,and inductors, and active components such as p-type field effecttransistors (pFETs), n-type FETs (nFETs), FinFETs, MOSFETs, CMOS,bipolar transistors, high voltage transistors, high frequencytransistors, other memory cells, and combinations thereof.

FIGS. 1A-1C are flow charts of an example method for fabricating anembodiment of a GAA device of the present disclosure according to someembodiments of the present disclosure. FIGS. 2A-29A are top views of anembodiment of a GAA device of the present disclosure constructed atvarious fabrication stages according to some embodiments of the presentdisclosure. FIGS. 2B-29B, 2C-29C, and 2D-29D are cross sectional viewsof an embodiment of a GAA device of the present disclosure along thelines A-A′, B-B′, and C-C′ in FIGS. 2A-29A, respectively, according tosome embodiments of the present disclosure.

Referring to block 810 of FIG. 1A and FIGS. 2A-2D, the GAA device 100includes a substrate 200. In some embodiments, the substrate 200contains a semiconductor material, such as bulk silicon (Si).Alternatively or additionally, another elementary semiconductor, such asgermanium (Ge) in a crystalline structure, may also be included in thesubstrate 200. The substrate 200 may also include a compoundsemiconductor, such as silicon germanium (SiGe), silicon carbide (SiC),gallium arsenic (GaAs), gallium phosphide (GaP), indium phosphide (InP),indium arsenide (InAs), and/or indium antimonide (InSb), or combinationsthereof. The substrate 200 may also include a semiconductor-on-insulatorsubstrate, such as Si-on-insulator (SOI), SiGe-on-insulator (SGOI),Ge-on-insulator (GOI) substrates. Portions of the substrate 200 may bedoped, such as the doped portions 205. The doped portions 205 may bedoped with p-type dopants, such as boron (B) or boron fluoride (BF₃), ordoped with n-type dopants, such as phosphorus (P) or arsenic (As). Thedoped portions 205 may also be doped with combinations of p-type andn-type dopants. The doped portions 205 may be formed directly on thesubstrate 200, in a p-well structure, in an n-well structure, in adual-well structure, or using a raised structure.

Referring to block 820 of FIG. 1A and FIGS. 2A-2D, a stack ofsemiconductor layers 220A and 220B are formed over the substrate 200 inan interleaving or alternating fashion, extending vertically (e.g. alongthe Z-direction) from the substrate 200. For example, a semiconductorlayer 220B is disposed over the substrate 200, a semiconductor layer220A is disposed over the semiconductor layer 220B, and anothersemiconductor layer 220B is disposed over the semiconductor layer 220A,so on and so forth. In the depicted embodiments, there are three layersof semiconductor layers 220A and three layers of semiconductor layers220B alternating between each other. However, there may be anyappropriate number of layers in the stack. For example, there may be2-10 layers of semiconductor layers 220A, alternating with 2-10 layersof semiconductor layers 220B in the stack. The material compositions ofthe semiconductor layers 220A and 220B are configured such that theyhave an etching selectivity in a subsequent etching process. Forexample, in some embodiments, the semiconductor layers 220A containsilicon germanium (SiGe), while the semiconductor layers 220B containsilicon (Si). In some other embodiments, the semiconductor layers 220Bcontain SiGe, while the semiconductor layers 220A contain Si. In thedepicted embodiment, the semiconductor layers 220A each has asubstantially uniform thickness, referred to as the thickness 300, whilethe semiconductor layers 220B each has a substantially uniformthickness, referred to as the thickness 310.

Referring to block 820 of FIG. 1A and FIGS. 3A-3D, the stack ofsemiconductor layers 220A and 220B are patterned into a plurality of finstructures, for example, into fin structures (or fins) 130 a and 130 b.Each of the fins 130 a and 130 b includes a stack of the semiconductorlayers 220A and 220B disposed in an alternating manner with respect toone another. The fins 130 a and 130 b each extends lengthwisehorizontally in a Y-direction and are separated from each otherhorizontally in an X-direction, as shown in FIGS. 3A and 3D. Asillustrated in FIG. 3A, the fins may each have a lateral width along theX-direction, referred to as the width 350. It is understood that theX-direction and the Y-direction are horizontal directions that areperpendicular to each other, and that the Z-direction is a verticaldirection that is orthogonal (or normal) to a horizontal XY planedefined by the X-direction and the Y-direction. The semiconductorsubstrate may have its top surface aligned in parallel to the XY plane.

The fins 130 a and 130 b may be patterned by any suitable method. Forexample, the fins may be patterned using one or more photolithographyprocesses, including double-patterning or multi-patterning processes.Generally, double-patterning or multi-patterning processes combinephotolithography and self-aligned processes, allowing patterns to becreated that have, for example, pitches smaller than what is otherwiseobtainable using a single, direct photolithography process. For example,in one embodiment, a sacrificial layer is formed over a substrate andpatterned using a photolithography process. Spacers are formed alongsidethe patterned sacrificial layer using a self-aligned process. Thesacrificial layer is then removed, and the remaining spacers, ormandrels, may then be used to pattern the fins. The patterning mayutilize multiple etching processes which may include a dry etchingand/or wet etching. The regions in which the fins are formed will beused to form active devices through subsequent processing, and are thusreferred to as active regions. For example, fin 130 a is formed in theactive region 202 a, and the fin 130 b is formed in the active region202 b. Both fins 130 a and 130 b are formed over the doped portions 205.

The structure 100 includes isolation features 203, which may be shallowtrench isolation (STI) features. In some examples, the formation of theisolation features 203 includes etching trenches into the substrate 200between the active regions and filling the trenches with one or moredielectric materials such as silicon oxide, silicon nitride, siliconoxynitride, other suitable materials, or combinations thereof. Anyappropriate methods, such as a chemical vapor deposition (CVD) process,an atomic layer deposition (ALD) process, a physical vapor deposition(PVD) process, a plasma-enhanced CVD (PECVD) process, a plasma-enhancedALD (PEALD) process, and/or combinations thereof may be used fordepositing the isolation features 203. The isolation features 203 mayhave a multi-layer structure such as a thermal oxide liner layer overthe substrate 200 and a filling layer (e.g., silicon nitride or siliconoxide) over the thermal oxide liner layer. Alternatively, the isolationfeatures 203 may be formed using any other traditional isolationtechnologies. As illustrated in FIG. 3D, the fins 130 a and 130 b arelocated above the top surface of the isolation features 203 and the topsurface of the doped portions 205.

Referring to block 830 of FIG. 1A and FIGS. 4A-4D, dummy gate structures210 are formed over a portion of each of the fins 130 a and 130 b, andover the isolation features 203 in between the fins 130 a and 130 b. Thedummy gate structures 210 may be configured to extend lengthwise inparallel to each other, for example, each along the X-direction. In someembodiments, as illustrated in FIG. 4D, the dummy gate structures eachwraps around the top surface and side surfaces of each of the fins. Thedummy gate structures 210 may include polysilicon. In some embodiments,the dummy gate structures 210 also include one or more mask layers,which are used to pattern the dummy gate electrode layers. The dummygate structures 210 may undergo a gate replacement process throughsubsequent processing to form metal gates, such as a high-k metal gate,as discussed in greater detail below. Some of the dummy gate structures210 may also undergo a second gate replacement process to form adielectric based gate that electrically isolates the GAA device 100 fromits neighboring devices, as also discussed in greater detail below. Thedummy gate structures 210 may be formed by a procedure includingdeposition, lithography patterning, and etching processes. Thedeposition processes may include CVD, ALD, PVD, other suitable methods,and/or combinations thereof.

Referring to block 840 of FIG. 1A and FIGS. 5A-5D, gate spacers 240 areformed on the sidewalls of the dummy gate structures 210. The gatespacers 240 may include silicon nitride (Si₃N₄), silicon oxide (SiO₂),silicon carbide (SiC), silicon oxycarbide (SiOC), silicon oxynitride(SiON), silicon oxycarbon nitride (SiOCN), carbon doped oxide, nitrogendoped oxide, porous oxide, or combinations thereof. The gate spacers 240may include a single layer or a multi-layer structure. In someembodiments, the gate spacers 240 may have thicknesses in the range of afew nanometers (nm). In some embodiments, the gate spacers 240 may beformed by depositing a spacer layer (containing the dielectric material)over the dummy gate structures 210, followed by an anisotropic etchingprocess to remove portions of the spacer layer from the top surfaces ofthe dummy gate structures 210. After the etching process, portions ofthe spacer layer on the sidewall surfaces of the dummy gate structures210 substantially remain and become the gate spacers 240. In someembodiments, the anisotropic etching process is a dry (e.g. plasma)etching process. Additionally or alternatively, the formation of thegate spacers 240 may also involve chemical oxidation, thermal oxidation,ALD, CVD, and/or other suitable methods. In the active regions, the gatespacers 240 are formed over the top layer of the semiconductor layers220A and 220B. Accordingly, the gate spacers 240 may also beinterchangeably referred to as the top spacers 240. In some examples,one or more material layers (not shown) may also be formed between thedummy gate structures 210 and the corresponding top spacers. The one ormore material layers may include an interfacial layer and/or a high-kdielectric layer.

Referring to block 850 of FIG. 1A and FIGS. 6A-6D, portions of the fins130 a and 130 b exposed by the dummy gate structures 210 (e.g. in thesource/drain regions) are at least partially recessed (or etched away)to form the source/drain trenches 151 for subsequent epitaxial sourceand drain growths. Meanwhile, the portions underneath the dummy gatestructures 210 remain intact. In the depicted embodiments, the exposedportions of the fins 130 a and 130 b are etched away entirely.Accordingly, the top surface of the doped portions 205 of the substrate200 in the source/drain regions are exposed in the source/drain trenches151. The top surface of the doped portions 205 (and the bottom surfaceof the source/drain trenches 151) extends along the top surface of thesubstrate 200. Alternatively, in some other embodiments (not shown), therecess process removes only some, but not all, of the semiconductorlayers 220A and 220B not covered by the dummy gate structures 210. Inother words, the bottom surface of the source/drain trenches 151 extendabove the top surface of the substrate 200. In yet some otherembodiments (not shown), the recess process may remove not only theexposed fins 130 a and 130 b, but also remove a portion of theunderlying doped region 205. In other words, the top surface of thedoped portions 205 (and the bottom surface of the source/drain trenches151) extends below the top surface of the substrate 200.

In the depicted embodiments, as illustrated in FIG. 6B, the remainingstack of semiconductor layers 220A and 220B are only present verticallybeneath the dummy gate structures 210 (referred to as the “centerportions”) and vertically beneath the top spacers 240 (referred to asthe “end portions” or interchangeably “side portions”). Accordingly, theportion of the semiconductor layers 220A vertically beneath the dummygate structures 210 are referred to as the center portions 220A-center;while the portions of the semiconductor layers 220A vertically beneaththe top spacers 240 are referred to as the end portions 220A-end.Similarly, the portion of the semiconductor layers 220B verticallybeneath the dummy gate structures 210 are referred to as the centerportions 220B-center; while the portions of the semiconductor layers220B vertically beneath the top spacers 240 are referred to as the endportions 220B-end. In other words, the top spacers 240 are formed overthe two end portions 220A-end of the topmost layer of the semiconductorlayers 220A. The recess process may include multiple lithography andetching steps, and may use any suitable methods, such as dry etchingand/or wet etching.

The formation of the source/drain trenches 151 exposes the sidewalls ofthe stack of semiconductor layers 220A and 220B. Referring to block 860of FIG. 1A and FIGS. 7A-7D, portions of the semiconductor layers 220Bare removed through the exposed sidewall surfaces in the trenches 151via a selective etching process. The selective etching process may beany suitable processes, such as a wet etching or a dry etching process.The extent to which the semiconductor layers 220B are recessed (or thesize of the portion removed) is determined by the processing conditionssuch as the duration the semiconductor layers 220B is exposed to anetching chemical. In the depicted embodiments, the duration iscontrolled such that the end portions 220B-end are removed in theirentirety, while the center portions 220B-center remain substantiallyunchanged. In other words, the remaining portions of the semiconductorlayers 220B each has a sidewall that extends along a sidewall of thedummy gate structures 210 (e.g. the sidewall in the XZ plane, defined bythe X-direction and the Z-direction). As illustrated in FIG. 7B, theselective etching process creates openings 161, which extend thetrenches 151 into areas beneath the semiconductor layers 220A and topspacers 240.

Meanwhile, the semiconductor layers 220A are only slightly affectedduring the selective etching process. For example, prior to theselective etching process, the end portions 220A-end each has athickness 300, and end portions 220B-end each has a thickness 310 (seeFIG. 6B). After the selective etching process, the end portions 220A-endhas a thickness 305, and the openings 161 has a height 315 (orinterchangeably referred to as thickness 315). Thickness 305 is onlyslightly smaller than thickness 300; and thickness 315 is only slightlylarger than thickness 310. For example, thickness 305 may be about 1% to10% smaller than thickness 300; and thickness 315 may be about 1% to 10%larger than thickness 310. As discussed above, the selectivity betweenthe semiconductor layers 220A and 220B is made possible due to thedifferent material compositions between these layers. For example, thesemiconductor layers 220A may be etched away at a substantially fasterrate (e.g. more than about 5 to about 10 times faster) than thesemiconductor layers 220B.

As discussed above, the selective etching process may be a wet etchingprocess. In an embodiment, the semiconductor layers 220A includes Si andthe semiconductor layers 220B includes SiGe. A Standard Clean 1 (SC-1)solution may be used to selectively etch away the SiGe semiconductorlayers 220B. For example, the SiGe semiconductor layers 220B may beetched away at a substantially faster rate than the Si semiconductorlayers 220A. As a result, desired portions of the semiconductor layers220B, e.g. the end portions 220B-end, are removed while thesemiconductor layers 220A remain substantially unchanged. The SC-1solution includes ammonia hydroxide (NH₄OH), hydrogen peroxide (H₂O₂),and water (H₂O). The etching duration is adjusted such that the size ofthe removed portions of SiGe layers are controlled. The optimalcondition may be reached by additionally adjusting the etchingtemperature, dopant concentration, as well as other experimentalparameters.

In another embodiment, the semiconductor layers 220A include SiGe andthe semiconductor layers 220B includes Si. A cryogenic deep reactive ionetching (DRIE) process may be used to selectively etch away the Sisemiconductor layer 220B. For example, the DRIE process may implement asulfur hexafluoride-oxygen (SF₆—O₂) plasma. The optimal condition may bereached by adjusting the etching temperature, the power of theInductively Coupled Plasma (ICP) power source and/or Radio Frequency(RF) power source, the ratio between the SF₆ concentration and the O₂concentration, the dopant (such as boron) concentrations, as well asother experimental parameters. For example, the etching rate of a Sisemiconductor layer 220B using a SF₆—O₂ plasma (with approximately 6%O₂) may exceed about 8 μm/min at a temperature of about −80° C.; whilethe SiGe semiconductor layers 220A are not substantially affected duringthe process.

Referring to block 870 of FIG. 1A and FIGS. 8A-8D, a dielectric material248 is deposited into both the source/drain trenches 151 and theopenings 161. The dielectric material 248 may be selected from SiO₂,SiON, SiOC, SiOCN, or combinations thereof. In some embodiments, theproper selection of the dielectric material 248 may be based on itsdielectric constant. In an embodiment, this dielectric material 248 mayhave a dielectric constant lower than that of the top spacers 240. Insome other embodiments, this dielectric material 248 may have adielectric constant higher than that of the top spacers 240. This aspectof the dielectric material 248 will be further discussed later. Thedeposition of the dielectric material 248 may be any suitable methods,such as CVD, PVD, PECVD, MOCVD, ALD, PEALD, or combinations thereof. ACMP process may be performed to planarize the top surfaces of the device100, and to expose the top surfaces of the dummy gate structures 210.

Referring to block 880 of FIG. 1A and FIGS. 9A-9D, the dielectricmaterial 248 is etched back such that the top surface of the dopedregions 205 is exposed. In the depicted embodiment, the etching-back isa self-aligned anisotropic dry-etching process, such that the topspacers 240 are used as the masking element. Alternatively, a differentmasking element (e.g. a photoresist) may be used. The etching-backcompletely removes the dielectric materials 248 within the trenches 151but does not substantially affect the dielectric materials 248 withinthe trenches 161 (see FIG. 8B). As a result, the dielectric materials248 filled in the openings 161 become the inner spacers 250. In otherwords, the inner spacers 250 are formed between vertically adjacent endportions 220A-end of the semiconductor layers 220A (see FIG. 9B). In thepresent embodiment, the inner spacers 250 are only present in the activeregions. As illustrated in FIG. 9C, no inner spacers 250 are presentover the isolation features 203. Rather, only top spacers 240 arepresent over the isolation features 203.

As illustrated in FIG. 9B, the sidewall surfaces of the inner spacers250, the top spacers 240, and the semiconductor layers 220A formcontinuous sidewall surfaces 171. The side wall surfaces 171 includeboth semiconductor materials from the semiconductor layers 220A anddielectric materials from the top spacers 240 and the inner spacers 250.In an embodiment, the semiconductor layers 220A may have substantiallymaintained the thickness 305, while the inner spacers 250 may have aboutthe same thickness 315 as the openings 161 (see FIG. 7B). As describedabove, thickness 305 is largely determined by thickness 300, and isslightly smaller (such as 1% to 10% smaller) than the thickness 300.Similarly, thickness 315 is largely determined by thickness 310, and isslightly larger (such as 1% to 10% larger) than thickness 310. The ratior₁, defined by thickness 305 over thickness 315, may be used incontrolling the optimization of epitaxial source/drain features. This isdescribed in more details below.

Referring to block 890 of FIG. 1B and FIGS. 10A-10D, the method 800continues to forming epitaxial source/drain features 208 in thesource/drain trenches 151. In some embodiments, a source/drain featureis a source electrode, and the other source/drain feature is a drainelectrode. As illustrated in FIG. 10B, each of the semiconductor layers220A connects two epitaxial source/drain features. A portion of thesemiconductor layers 220A may constitute a portion of a transistorchannel. Multiple processes including etching and growth processes maybe employed to grow the epitaxial source/drain features 208. In thedepicted embodiment, the epitaxial source/drain features 208 have topsurfaces that extend higher than the top surface of the topmostsemiconductor layer 220A. However, in other embodiments, the epitaxialsource/drain features 208 may alternatively have top surfaces that areabout even with the top surface of the topmost semiconductor layer 220A.In the depicted embodiment, the epitaxial source/drain features 208occupy a lower portion of the trenches 151, leaving an upper portion ofthe trenches 151 open.

In some embodiments, the epitaxial source/drain features 208 may mergetogether, for example, along the X-direction, to provide a largerlateral width than an individual epitaxial feature. In the depictedembodiments, the epitaxial source/drain features 208 are not merged. Theepitaxial source/drain features 208 may include any suitablesemiconductor materials. For example, the epitaxial source/drainfeatures 208 in an n-type GAA device may include Si, SiC, orcombinations thereof; while the epitaxial source/drain features 208 in ap-type GAA device may include Si, SiGe, Ge, SiGeC, or combinationsthereof. The source/drain features 208 may be doped in-situ or ex-situ.For example, the epitaxially grown Si source/drain features may be dopedwith carbon to form silicon:carbon (Si:C) source/drain features,phosphorous to form silicon:phosphor (Si:P) source/drain features, orboth carbon and phosphorous to form silicon carbon phosphor (SiCP)source/drain features; and the epitaxially grown SiGe source/drainfeatures may be doped with boron. One or more annealing processes may beperformed to activate the dopants in the epitaxial source/drain features208. The annealing processes may comprise rapid thermal annealing (RTA)and/or laser annealing processes.

The epitaxial source/drain features 208 directly interface with thesidewalls 171, which include sidewalls of the semiconductor layers 220A,of the inner spacers 250, and possibly of the top spacers 240 (dependingon the height of the epitaxial source/drain features 208). During theepitaxial growth, semiconductor materials grow from the exposed topsurface of the doped region 205 as well as from the exposed surface ofthe semiconductor layers 220A and do not grow on the surfaces of theinner spacers 250 and the top spacers 240. Therefore, it is desirable tominimize the surface area of the inner spacers 250 (and top spacers 240)on the sidewall surface 171. In other words, a smaller thickness of theinner spacer 250 (e.g. thickness 315) along the Z-direction, and alarger thickness of the semiconductor layer 200A (e.g. thickness 305)are favorable for the growth and the quality of the epitaxialsource/drain features. The inventor of the present disclosure hasdiscovered that a ratio r₁ (defined as the ratio of thickness 305 tothickness 315) higher than 0.9 results in good crystalline quality inthe epitaxial source/drain features 208. In some embodiment, the ratior₁ is set to be in a range of 0.9 to 2.5. In some other embodiments, theratio r₁ is set to be in a range of 1 to 2 for obtaining highcrystalline quality in the epitaxial source/drain features 208.

Referring to block 900 of FIG. 1B and FIGS. 11A-11D, an interlayerdielectric (ILD) layer 214 is formed over the epitaxial source/drainfeatures 208 in the remaining spaces of the trenches 151, as well asvertically over the isolation features 203. The ILD layer 214 may alsobe formed in between the adjacent gates 210 along the Y-direction, andin between the source/drain features 208 along the X-direction. The ILDlayer 214 may include a dielectric material, such as a high-k material,a low-k material, or an extreme low-k material. For example, the ILDlayer 214 may include SiO₂, SiOC, SiON, or combinations thereof. The ILDlayer 214 may include a single layer or multiple layers, and may beformed by a suitable technique, such as CVD, ALD, and/or spin-ontechniques. After forming the ILD layer 214, a CMP process may beperformed to remove excessive ILD layer 214 and planarized the topsurface of the ILD layer 214. Among other functions, the ILD layer 214provides electrical isolation between the various components of the GAAdevice 100.

Referring to block 910 of FIG. 1B and FIGS. 12A-12D, the dummy gatestructures 210 are selectively removed through any suitable lithographyand etching processes. In some embodiments, the lithography process mayinclude forming a photoresist layer (resist), exposing the resist to apattern, performing a post-exposure bake process, and developing theresist to form a masking element, which exposes a region including thedummy gate structures 210. Then, the dummy gate structures 210 areselectively etched through the masking element. In some otherembodiments, the top spacers 240 may be used as the masking element or apart thereof. For example, the dummy gate structures 210 may includepolysilicon, the top spacers 240 and the inner spacers 250 may includedielectric materials, and the semiconductor layers 220A-center includesa semiconductor material. Therefore, an etch selectivity may be achievedby selecting appropriate etching chemicals, such that the dummy gatestructures 210 may be removed without substantially affecting the othercomponents of the device 100.

The removal of the dummy gate structures 210 creates trenches 153. Thetrenches 153 expose the top surfaces and the side surfaces of the stack(along the X direction). In other words, the center portions 220A-centerand 220B-center are exposed at least on two side surfaces in thetrenches 153. Additionally, the trenches 153 also expose the topsurfaces of the isolation features 203. At this stage, the centerportions 220A-center and 220B-center may have lateral widths that aresubstantially similar to that of the fins 130 a and 130 b. Accordingly,the center portions 220A-center may also each have the thickness 300,and the center portions 220B-center may each have the thickness 310.

Referring to block 910 of FIG. 1B and FIGS. 13A-13D, the remainingcenter portions 220B-center are also selectively removed through thetrenches 153, for example using wet or dry etching process. The etchingchemical is selected such that the center portions 220B-center has asufficiently different etching rate as compared to the center portions220A-center and the inner spacers 250. As a result, the center portions220A-center and the inner spacers 250 are only slightly affected duringthe selective etching process. For example, after the etching process,the center-portions 220A-center each has a thickness 308. Thickness 308is slightly smaller than thickness 300, for example, 1% to 10% smallerthan thickness 300. In an embodiment, thickness 308 is substantiallyequal to thickness 305 of the end portions 220A-end. In other words, thesemiconductor layers 220A has a substantially uniform thickness at thisstage. Additionally, the selective etching process creates openings 157.The openings 157 each has a height 318 (or interchangeably referred toas thickness 318). Thickness 318 is slightly larger than thickness 310,for example, 1% to 10% larger than thickness 310. In an embodiment,thickness 318 is substantially equal to thickness 315 of the innerspacers 250. This selective etching process may include one or moreetching steps.

As illustrated in FIGS. 13A-13D, in the present embodiment, the removalof the semiconductor layers 220B forms suspended semiconductor layers220A-center and openings 157 in between the vertically adjacent layers,exposing the top and bottom surfaces of the center portions 220A-center.Each of the center portions 220A-center are now exposedcircumferentially around the Y-direction. In addition, the portion ofthe doped regions 205 beneath the center portions 220A-center are alsoexposed in the openings 157. In some other embodiments however, theremoval process only removes some but not all of the center portions220B-center. As a result, some of the center portions 220A-center areexposed circumferentially around the Y-direction, while others areexposed only on the two side surfaces along the X-direction; and thedoped regions 205 is not exposed.

As described above, it is desirable to increase the thickness of thesemiconductor layers 220A (or more specifically, the end portions220A-end) to improve the quality of the epitaxial source/drain features208. On the other hand, a thicker semiconductor layer 220A in thechannel region would reduce the available spacing between thesemiconductor layers to form other layers, such as the high-k gatedielectric layers, the metal layers, etc. Thus, the semiconductor layers220A are purposefully designed according to the present disclosure tohave different thicknesses and/or different widths in different regionsto address the above concerns. For example, the center portions220A-center of the semiconductor layers 220A each has a smallerthickness to allow sufficient processing margins for subsequentdepositions, while the end portions 220A-end each has a larger thicknessto effectuate good epitaxial growth of the epitaxial source/drainfeatures 208.

Referring to block 920 of FIG. 1B and FIGS. 14A-14D and, the centerportions 220A-center of the semiconductor layers 220A are subject to apartial etching treatment (or “thinning treatment”). This treatmentreduces the thickness of the center portions 220A-center. In otherwords, the center portions 220A-center of the semiconductor layers 220Aare circumferentially trimmed. As described above, the center portions220A-center and the end portions 220A-end have thickness 308 andthickness 305, respectively, prior to the partial etching treatment.Both thickness 308 and thickness 305 are slightly smaller (such as 1% to10% smaller) than thickness 300. In other words, the semiconductorlayers 220A have a substantially uniform thickness prior to thetreatment process. After the treatment however, while the end portions220A-end maintain thickness 305, the center portions 220A-center eachhas a thickness substantially smaller than thickness 308 (and thickness305). For example, as illustrated in the FIGS. 14B and 14D, the centerportions 220A-center now each has a thickness 320 that is substantiallysmaller than the end portions 220A-end (as described in more detailbelow). In some embodiments, both thicknesses 305 and 320 are within arange of about 3 nm to about 10 nm. If the thicknesses 305 and 320 aretoo smaller, such as smaller than 3 nm, the inevitable variations in thefabrication processes may result in unacceptable inconsistency amongdevices and ultimately negatively affect the process reliability.Conversely, if the thicknesses 305 and 320 are too large, such as largerthan 10 nm, gate control of the entire channel region may not bereached, and resistance during the operation may increase. Although notshown in the figures, the center portions 220A-center may also have awidth smaller than the end portions 220A-end along the X direction. Inother words, the lateral width of the semiconductor layers 220A-centerin FIG. 14A along the X-direction (width 360) may be smaller than thelateral width of the semiconductor layers 220A-center in FIG. 13A alongthe X-direction (width 350). In some embodiments, both the widths 350and 360 are within the range of about 6 nm to about 60 nm. Similar tothe variations in the thicknesses 305 and/or 320, if the width 350and/or 360 depart from this range, either the device reliability or thegate control of the channel region may be negatively affected.

The partial etching treatment may use any suitable methods, for example,a conformal wet-etching process. In an embodiment, the wet-etchingmethod implements an isotropic etching process with an acidic etchingchemical. For example, the etching chemical may include de-ionizedwater, ozone (O₃), and hydrofluoric acid, and the de-ionized water andhydrofluoric acid have a molar ratio between about 1:50 and about1:2000. In another embodiment, the wet-etching method implements anisotropic etching process with a basic etching chemical. For example,the etching chemical may include ammonia water. The duration of thetreatment in which the center portions 220A are exposed to the etchingchemical may be used to control the amount of the semiconductormaterials removed from the center portions 220A-center, therebycontrolling the thickness 320. Alternatively or additionally, thepartial etching treatment may use a conformal oxidation method. Forexample, a circumferential surface of the center portions 220A-centermay be subject to an oxidant, e.g. oxygen, to form a layer of oxides.The oxides are subsequently removed using any appropriate method, forexample, by exposure to an acid. As a result, the center portions220A-center has a reduced thickness as compared to before the treatment.

As described above, a smaller thickness 320 and a larger thickness 305is desirable. In an embodiment, the ratio r₂, defined by thickness 305over thickness 320, may be between about 1.1 to about 3. In anotherembodiment, the ratio r₂ may be between about 1.2 to about 2.Furthermore, in an embodiment, the difference Δ between thickness 305and thickness 320 is larger than about 0.5 nm to about 3 nm. If theratio r₂ is too small, such as smaller than about 1.1, or the differenceΔ is too small, such as smaller than about 0.5 nm, the benefit forincreasing the surface area for epitaxial growth margin may not bediscernable. On the other hand, if the ratio r₂ is too large, such aslarger than about 3, the improvement in the epitaxial growth margin mayhave been saturated while processing challenges may increase.

Referring to FIGS. 15A-15D, a dielectric layer 223 is formed over thecenter portions 220A-center of the semiconductor layers 220A. Thisdielectric layer 223 may be an interfacial layer. Any suitable methodsmay be used to form the dielectric layer 223, such as ALD, CVD, or otherdeposition methods. Alternatively, in the depicted embodiments, thedielectric layer 223 may also be formed by an oxidation process, such asthermal oxidation or chemical oxidation (as seen in FIGS. 15A-15D). Inthis instance, no interfacial layer 223 is formed on the sidewalls ofthe top spacers 240 or the inner spacers 250. In many embodiments, theinterfacial layer 223 improves the adhesion between the semiconductorsubstrate and the subsequently formed gate dielectric layers. In someembodiments, the interfacial layers 223 are omitted.

Referring to blocks 930-940 of FIG. 1B, FIGS. 16A-16D, and FIGS.17A-17D, a gate structure is formed. The gate structure includes a gatedielectric layer and a gate electrode over the gate dielectric layer.For example, the gate structure may include a polysilicon gate electrodeover a SiON gate dielectric layer. For another example, the gatestructure may include a metal gate electrode over a high-k dielectriclayer. In some instances, a refractory metal layer may interpose betweenthe metal gate electrode (such as an aluminum gate electrode) and thehigh-k dielectric layer. For yet another example, the gate structure mayinclude silicide. In the depicted embodiment, the gate structures eachincludes a high-k gate dielectric layer 228 and a gate electrode thatcomprises one or more metal layers 230-232. The high-k gate dielectriclayers 228 are formed between the metal layers 230-232 and thenanochannels of the semiconductor layers 220A (e.g. the center portions220A-center).

In some embodiments, the high-k gate dielectric layers 228 are formedconformally on the device 100 (see FIGS. 16A-16D). This high-k gatedielectric layers 228 at least partially fill the trenches 153. In someembodiments, the high-k gate dielectric layers 228 may be formed aroundthe exposed surfaces of each of the semiconductor layers 220A, such thatthey wrap around the center portions 220A-center of each of thesemiconductor layers 220A in 360 degrees. The high-k gate dielectriclayers 228 also directly contact the vertical sidewalls of theinterfacial layer 223, or in its absence, directly contact the verticalsidewalls of the two end portions 220A-end of each of the semiconductorlayers 220A. Additionally, the high-k gate dielectric layers 228 alsodirectly contact vertical sidewalls of the inner spacers 250 and of thetop spacers 240. The high-k gate dielectric layers 228 contain adielectric material having a dielectric constant greater than adielectric constant of SiO₂, which is approximately 3.9. For example,the high-k gate dielectric layers 228 may include hafnium oxide (HfO₂),which has a dielectric constant in a range from about 18 to about 40. Asvarious other examples, the high-k gate dielectric layers 228 mayinclude ZrO₂, Y₂O₃, La₂O₅, Gd₂O₅, TiO₂, Ta₂O₅, HfErO, HfLaO, HfYO,HfGdO, HfAlO, HfZrO, HMO, HfTaO, SrTiO, or combinations thereof. Theformation of the high-k gate dielectric layers 228 may be by anysuitable processes, such as CVD, PVD, ALD, or combinations thereof.

Referring to block 940 of FIG. 1B and FIGS. 17A-17D, the metal layers230-232 are formed over the high-k gate dielectric layers 228 and fillthe remaining spaces of the trenches 153. The metal layers 230-232 mayinclude any suitable materials, such as titanium nitride (TiN), tantalumnitride (TaN), titanium aluminide (TiAl), titanium aluminum nitride(TiAlN), tantalum aluminide (TaAl), tantalum aluminum nitride (TaAlN),tantalum aluminum carbide (TaAlC), tantalum carbonitride (TaCN),aluminum (Al), tungsten (W), copper (Cu), cobalt (Co), nickel (Ni),platinum (Pt), or combinations thereof. In some embodiments, a CMP isperformed to expose a top surface of the ILD 214. The dielectric layers228 and the metal layers 230 collectively form the high-k metal gates(HKMG) 270 and the dielectric layers 228 and the metal layers 232collectively form the high-k metal gates (HKMG) 272. The HKMG 270-272each engages multiple nanochannels, e.g. multiple layers within thecenter portions 220A-center.

In some embodiments, a gate top hard mask layer 260 may optionally beformed over the HKMG 270-272. For example, referring to FIGS. 18A-18D,the metal layers 230-232 may optionally be recessed, such that a topsurface of the metal layer 230-232 extends below a top surface of theILD 214. Subsequently, as illustrated in FIGS. 19A-19D, a gate top hardmask layer 260 is formed over the GAA device 100 such that it covers theHKMG 270-272 (specifically, the metal layers 230-232), the ILD layers214, and fills the space created by the recess process. A CMP may beconducted to planarize the top surfaces. In some embodiments, asillustrated in FIGS. 20A-20D, the CMP exposes the top surfaces of theILD layers 214, the top surfaces of the top spacers 240, and the topsurface of the gate top hard mask layer 260. The gate top hard masklayers 260 may include a dielectric material, such as SiO₂, SiOC, SiON,SiOCN, nitride-based dielectric, metal oxide dielectric, HfO₂, Ta₂O₅,TiO₂, ZrO₂, Al₂O₃, Y₂O₃, or combinations thereof. The gate top hard masklayer 260 protects the HKMG 272 in the subsequent etching processes toform the source/drain contact features, and also insulates the HKMG 272.However, in some other embodiments (not shown), the recess of the metallayers 230-232 and/or the formation of the gate top hard mask layers 260is omitted.

Referring to block 950 of FIG. 1C and FIGS. 21A-21D, a mask layer 282(e.g. a photoresist layer) is formed over the top surface of the device100. The mask layer 282 may cover the main body (or the center region)of the device 100 but not the two end regions 283 (along theX-direction) of the device 100.

Referring to block 960 of FIGS. 1C and 22A-22D, an end-cut process issubsequently conducted. The end-cut process forms end-cut trenches 155(see FIGS. 22A-22D), which split the HKMG 270-272 along the X directioninto individual gates. The individual gates may extend over an n-typeregion only, over a p-type region only, or over both an n-type regionand a p-type region. The end-cut process may include any suitablelithography and etching processes such that the end regions 283 areetched down to expose the isolation structure 203.

Referring to block 970 of FIG. 1C and FIGS. 23A-23D, a dielectricmaterial is deposited into the end-cut trenches 155 to form the gate enddielectric layer 262, which extends from a top surface of the isolationfeatures 203 and fully covers an end of the gates, such as the HKMG270-272. The gate end dielectric layer 262 may include a nitride-baseddielectric material (e.g., Si₃N₄), a metal oxide, SiO₂, or combinationsthereof. As described later, a subsequent step removes the top spacers240 and the inner spacers 250 without substantially affecting the gateend dielectric layer 262. Therefore, there needs to be sufficientetching selectivity between the gate end dielectric layer 262 and thespacer layers (240 and 250). For example, the etching rate for the topspacers 240 and the inner spacers 250 in the etching chemical may besubstantially faster than the etching rate for the gate end dielectriclayer 262 in the same solution, e.g. more than about 5 to 50 timesfaster. This difference in etching rate is a result of the differentcharacteristics of the materials in these different layers, which mayalso be manifested in their different dielectric constants. In manyembodiments, the gate end dielectric material may have a dielectricconstant higher than both that of the top spacers 240 and that of theinner spacers 250. For example, the gate end dielectric layer 262 mayinclude a dielectric material with a dielectric constant larger thanabout 6.9 to about 7. For example, the gate end dielectric layer 262 mayinclude nitride. The nitride may have a dielectric constant larger thanabout 7.8 to about 8.0. On the other hand, the top spacers 240 and/orthe inner spacers 250 may include oxide-based dielectric materials. Forexample, the top spacers 240 and/or the inner spacers 250 may includeoxides with a dielectric constant in the range of about 3.9 to about5.0. For another example, the top spacers 240 and/or the inner spacers250 may include doped oxides, such as nitrogen-doped oxides and/orcarbon-doped oxides. The nitrogen-doped oxide may have a dielectricconstant between about 4 and about 5. The carbon-doped oxide may have adielectric constant between about 3 and about 4. In some embodiments,the gate end dielectric layer 262 may include a single layer. In someother embodiments, the gate end dielectric layer 262 may includemultiple layers, such as a nitride layer and an oxide layer.

Referring to block 980 of FIG. 1C and FIGS. 24A-24D, a mask layer 284(e.g. a photoresist layer) is formed over the GAA device 100. In anembodiment, the mask layer 284 covers one or more HKMG 272 but notcovering one or more HKMG 270. Subsequently, referring to block 990 ofFIG. 1C and FIGS. 25A-25D, the exposed HKMG 270 are removed via anysuitable processes to form gate trenches 157. As a result, the dopedregions 205 as well as the isolation features 203 beneath the HKMG 270are exposed in the gate trenches 157. The etching process may be a wetetching or a dry etching process, using the mask layer 284 as themasking elements. In the depicted embodiment, the etching process notonly removes the exposed HKMG 270, but also removes the gate dielectriclayer 228 and partially recesses the doped region 205 of the substrate200. However, in other embodiments, the removal of the gate dielectriclayer 228 and/or the recess of the doped region 205 may be omitted.Alternatively or additionally, the sidewalls of the top spacers 240 maybe used as masking elements.

As illustrated in block 1000 of FIG. 1C and FIGS. 26A-26D, the gatetrenches 157 are filled with one or more dielectric materials to formthe dielectric based gates 234. At this stage, some of the end portions220A-end are located between and connect the dielectric based gates 234and the epitaxial source/drain features 208. These end portions 220A-endmay also be referred to as “wing portions” of the semiconductor layers220A. The dielectric materials may include SiO₂, SiOC, SiON, SiOCN,carbon-doped oxide, nitrogen-doped oxide, carbon-doped andnitrogen-doped oxide, dielectric metal oxides such as HfO₂, Ta₂O₅, TiO₂,ZrO₂, Al₂O₃, Y₂O₃, lanthanum- (La-) doped oxide, oxide doped withmultiple metals, or combinations thereof. The dielectric based gates 234may include a single layer or multiple layers. The formation processesmay use any suitable processes, such as ALD, CVD, PVD, PEALD, PECVD, orcombinations thereof. A CMP process may be performed to remove excessivedielectric materials and provide a top surface that is substantiallycoplanar with the ILD layer 214, the top spacers 240, and the gate enddielectric layers 262.

Referring to block 1010 of FIG. 1C and FIGS. 27A-27D, a gate topdielectric layer 290 is formed over the GAA device 100. The gate topdielectric layer 290 may be formed by any suitable processes, such asCVD, PECVD, flowable CVD (FCVD), or combinations thereof. The gate topdielectric layer 290 covers top surfaces of the dielectric based gates234, the ILD 214, the top spacers 240, the HKMG 272, and the gate tophard mask layer 260, if present. The gate top dielectric layer 290 mayinclude a dielectric material, such as SiO₂, SiOC, SiON, SiOCN,nitride-based dielectric, metal oxide dielectric, HfO₂, Ta₂O₅, TiO₂,ZrO₂, Al₂O₃, Y₂O₃, or combinations thereof. The gate top dielectriclayer 290 may have a thickness between about 3 nm and about 30 nm. Insome embodiments, the gate top dielectric layer 290 protect the HKMG 272in the subsequent etching processes to form the source/drain contactfeatures, and also insulate the HKMG 272.

Referring to block 1020 of FIG. 1C and FIGS. 28A-28D, a portion of thegate top dielectric layer 290 and ILD 214 are removed to form contactholes 278 over the epitaxial source/drain features 208. Any appropriatemethods may be used to form the contact holes 278, such as multiplelithography and etching steps. In an embodiment, a self-aligned contactformation process may be utilized. For example, the ILD 214 may includea dielectric material that has an etching rate substantially faster thanthat of the top spacers 240 and that of the gate top hard mask layer260. Therefore, the top spacers 240 and the gate top hard mask layer 260are not substantially affected when the ILD 214 is etched away to formthe contact holes 278. As the top spacers 240 and the gate top hard masklayer 260 protect the HKMG 272 from the etching chemical, the integrityof the HKMG 272 are preserved. The contact holes 278 expose the topsurfaces of the epitaxial source/drain features 208 for subsequentcontact layer formation. Additionally, a portion of the gate topdielectric layer 290 and the gate top hard mask layer 260 (if present)are also removed to form via holes 285 over the metal layers 232 of theHKMG 272. The via holes 285 expose the metal layers 232 for subsequentvia feature formation. Any appropriate methods may be used to form thevia holes 285, and may include multiple lithography and etching steps.

Referring to block 1030 of FIG. 1C and FIGS. 29A-29D, contact features280 are formed within the contact holes 278. Accordingly, the contactfeatures 280 are embedded within the gate top dielectric layer 290 andILD 214, and electrically connect the epitaxial source/drain features208 to external conductive features (not shown). Additionally, viafeatures 286 are also formed in the via holes 285. Accordingly, the viafeatures 286 are embedded within the gate top dielectric layer 290 (andwithin the gate top hard mask layer 260, if present) and electricallyconnect the HKMG 272 to external conductive features (not shown). Thecontact features 280 and the via features 286 may each include Ti, TiN,TaN, Co, Ru, Pt, W, Al, Cu, or combinations thereof, respectively. Anysuitable methods may be used to form the contact features 280 and thevia features 286. In some embodiments, additional features are formed inbetween the source/drain features 208 and the contacts 280, such asself-aligned silicide features 288. A CMP process may be performed toplanarize the top surface of the GAA device 100.

As discussed above, there needs to be a good etching selectivity betweenthe top spacers 240 and the inner spacers 250 in order to selectivelyremove the inner spacer material from the source/drain trench whileleaving the top spacer intact (block 870 of the FIG. 1A). Therefore,dielectric constants for the two spacers need to be different. Whetherthe top spacer or the inner spacer should use a material with a lowerdielectric constant may be a design choice. For example, the designchoice may be made based on a comparison between the relative importanceof the capacitance values of different device regions. The lower-kdielectric material may be assigned to increase the capacitance (orreduce the electronic coupling) of a particular region of the deviceaccording to the design needs.

More specifically, the top spacer 240 may be considered to be thedielectric medium of a capacitor between a pair of vertically alignedconductive plates, i.e., the sidewall of the contact 280 and thesidewall of the HKMG 272. Similarly, the inner spacer 250 may beconsidered to be the dielectric medium of another capacitor betweenanother pair of vertically aligned conductive plates, i.e. the sidewallof the source/drain feature 208 and the sidewall of the HKMG 272. Thecapacitance is proportional to the dielectric constant of the dielectricmedium, according to the following equation:

$C = {{ɛ\frac{A}{d}} = {kɛ_{0}\frac{A}{d}}}$

wherein C is the capacitance of the capacitor, c is the permittivity ofthe dielectric medium, ε₀ is the permittivity of vacuum, A is the areaof the capacitor, d is the separation distance of the capacitor, and kis the dielectric constant of the dielectric medium. Therefore, asmaller dielectric constant leads to a smaller capacitance. If,according to the design needs, it is more important to have a highercapacitance in the contact—metal gate region than in thesource/drain—metal gate region, the designer may assign the materialwith the lower k to the top spacer 240 rather than the inner spacer 250.On the other hand, if it is more important to have a higher capacitancein the source/drain-metal gate region, the designer may assign thematerial with the lower k to the inner spacer 250 rather than the topspacer 240.

Referring to block 1040 of FIG. 1C, additional layers and/or featuresmay also be formed above and/or within the gate top dielectric layer 290to complete the fabrication of the GAA device 100.

The above process flow describes one embodiment of the presentinvention. In this embodiment, the dielectric based gates 234 are formedafter the formation HKMG 270-272. However, the invention is not sorestricted, and other embodiments are also contemplated withoutdeparting from the spirit of the invention. For example, FIGS. 30A-38A,30B-38B, 30C-38C, and 30D-38D illustrate one alternative embodiment.Here, rather than forming the dielectric based gates 234 by removing theHKMG 270 and filling the gate trenches, they may alternatively be formedprior to the formation of the HKMG 270-272. In one such implementation,after the top spacers 240 are formed (e.g. as illustrated in FIGS.5A-5D), an ILD 304 may be formed over the GAA device 100. A mask layer384 may be formed over the ILD 304 to cover the entire area except theregion in which the dielectric based gates are to be formed (see FIGS.30A-30D). An etching process may be used to remove the exposed portionsof the ILD 304, as well as the dummy gate structures 210 beneath the ILD304. The etching process may also remove a portion of the doped regions205 under the dummy gate structures 210. This etching process formsdielectric base gate trenches 357, which are similar to those trenches157 illustrated in FIGS. 25A-25D. The mask layer 384 may then be removed(see FIGS. 31A-31D). Once the dielectric base gate trenches 357 areformed, a dielectric material, similar to those described above fordielectric base gates 234, are used to fill in the trenches 357 to formthe dielectric based gates 334 (see FIGS. 32A-32D). The method thenproceeds to conduct a CMP and etch a portion of the stack to form thesource/drain trenches 351, similar to those trenches 151 illustrated inthe FIGS. 6A-6D (see FIGS. 33A-33D). Subsequent processes may proceed inways similar to those illustrated in FIGS. 7A-23A, 7B-23B, 7C-23C, and7D-23D (see FIGS. 34A-37A, 34B-37B, 34C-37C, and 34D-37D). The finalstructure (see FIGS. 38A-38D) may be similar to that of FIGS. 29A-29D.

Additional details for this alternative embodiment may be found inrelated patents, such as U.S. Pat. No. 9,613,953, entitled“Semiconductor device, semiconductor device layout, and method ofmanufacturing semiconductor device” by Jhon Jhy Liaw, U.S. Pat. No.9,805,985, entitled “Method of manufacturing semiconductor device andsemiconductor device” by Jhon Jhy Liaw, and U.S. Pat. No. 9,793,273,entitled “Fin-based semiconductor device including a metal gatediffusion break structure with a conformal dielectric layer” by Jhon JhyLiaw. These patents are herein incorporated in their entities forreference.

Though not intended to be limiting, embodiments of the presentdisclosure offer benefits for semiconductor processing and semiconductordevices. For example, the disclosed method allows larger process marginsfor forming gate dielectric layers and metal layers within the limitedspacing between semiconductor channel layers of a GAA device than othertechnologies, thereby eliminating or reducing voids and/or other defectsin those layers. For a specific example, the nanochannels of the GAAdevices according to the present disclosure are purposefully thinned ascompared to the end regions of the semiconductor layers adjacent thenanochannels. This process enlarges the spacing available for materialdepositions, thereby improving the various aspects of the GAA devices.Additionally, the present method allows the semiconductor layers toinclude thicker end portions on which epitaxial source/drain featuresgrow. As a result, the epitaxial source/drain features are grown on sidesurfaces that include larger area of semiconductor materials, ratherthan dielectric materials. This improves the qualities of the epitaxialsource/drain features, and eventually improves the performance andreliability of the GAA device. Furthermore, this present method alsoprovides versatility allowing the designers to selectively optimize thecapacitances of different regions of the GAA device according to designneeds. As such, the present disclosure provides methods that improve theperformance, functionality, and/or reliability of GAA devices.

In an exemplary aspect, the present disclosure is directed to [Inventorand PE: I will rewrite the claims in paragraph form after PE approvesthe claims—thanks].

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An integrated circuit (IC) device, comprising: asemiconductor substrate having a top surface; a first source/drainfeature and a second source/drain feature over the semiconductorsubstrate; a first semiconductor layer extending in parallel with thetop surface and connecting the first source/drain feature and the secondsource/drain feature, the first semiconductor layer having a centerportion and two end portions, each of the two end portions connectingthe center portion and one of the first and the second source/drainfeatures; a first spacer over the two end portions of the firstsemiconductor layer; a second spacer vertically between the two endportions of the first semiconductor layer and the top surface; and agate electrode wrapping around and engaging the center portion of thefirst semiconductor layer; wherein the center portion of the firstsemiconductor layer has a first thickness, the two end portions of thefirst semiconductor layer each has a second thickness, and the firstthickness is smaller than the second thickness.
 2. The IC device ofclaim 1, further comprising a stack of semiconductor layers over thesemiconductor substrate, wherein the first semiconductor layer is atopmost layer of the stack of semiconductor layers; and each of thestack of semiconductor layers has a center portion vertically alignedwith the center portion of the first semiconductor layer and two endportions vertically aligned with the two end portions of the firstsemiconductor layer.
 3. The IC device of claim 2, wherein the centerportion of each of the stack of semiconductor layers has a thicknessabout equal to the first thickness, and the two end portions of each ofthe stack of semiconductor layers have a thickness about equal to thesecond thickness.
 4. The IC device of claim 1, further comprising: anisolation feature over the semiconductor substrate; and a gate enddielectric layer extending from a top surface of the isolation featureand covering an end of the gate electrode, wherein the first spacerincludes a first dielectric material with a first dielectric constant,the second spacer includes a second dielectric material with a seconddielectric constant, the gate end dielectric layer includes a thirddielectric material with a third dielectric constant, and wherein thethird dielectric constant is higher than the first and the seconddielectric constants.
 5. The IC device of claim 1, wherein thesemiconductor substrate includes a p-type region and an n-type region,wherein the gate electrode extends across the p-type region and then-type region.
 6. The IC device of claim 1, further comprising a dummygate electrode disposed on an opposite side of the first source/drainfeature from the gate electrode, the dummy gate electrode extending inparallel to the gate electrode, wherein a bottom surface of the dummygate electrode extends below a bottom surface of the gate electrode,wherein the first semiconductor layer includes a wing portion connectingthe first source/drain feature and the dummy gate electrode, the wingportion of the first semiconductor layer having a thickness about equalto the second thickness.
 7. The IC device of claim 1, wherein a ratio ofthe second thickness over the first thickness is between 1.1 and
 3. 8.The IC device of claim 7, wherein a difference between the secondthickness and the first thickness is at least 0.5 nm.
 9. The IC deviceof claim 7, wherein the first and the second thickness is each within arange of about 3 nm to about 10 nm.
 10. The IC device of claim 7,wherein the first semiconductor layer has a lateral width within a rangeof about 6 nm to about 60 nm.
 11. The IC device of claim 1, wherein aratio of the second thickness over the first thickness is between 1.2and
 2. 12. An integrated circuit (IC) device, comprising: asemiconductor substrate having a top surface; a first source/drainfeature and a second source/drain feature over the semiconductorsubstrate; semiconductor layers connecting the first source/drainfeature and the second source/drain feature, the semiconductor layersstacked over each other along a first direction normal to the topsurface, wherein each of the semiconductor layers has a center portionand two end portions, each of the two end portions connecting the centerportion and one of the first and the second source/drain features; agate electrode engaging the center portion of each of the semiconductorlayers; a first spacer over the two end portions of a topmostsemiconductor layer of the semiconductor layers; a second spacer betweenvertically adjacent end portions of the semiconductor layers; and a gateend dielectric layer contacting two ends of each of the semiconductorlayers; wherein the center portion of each of the semiconductor layershas a first thickness, the two end portions of each of the semiconductorlayers each has a second thickness, and the first thickness is smallerthan the second thickness, and wherein the gate electrode extends acrossa p-type doped region and an n-type doped region.
 13. The IC device ofclaim 12, wherein: the first spacer includes a first dielectric materialwith a first dielectric constant; the second spacer includes a seconddielectric material with a second dielectric constant; and the gate enddielectric layer includes a third dielectric material with a thirddielectric constant, the third dielectric constant being larger than thefirst and the second dielectric constants.
 14. The IC device of claim13, wherein: the first spacer includes a dielectric material selectedfrom SiO₂, SiON, SiOC, SiOCN, airgap, or combinations thereof; thesecond spacer includes a dielectric material selected from SiO₂, Si₃N₄,carbon doped oxide, nitrogen doped oxide, porous oxide, or combinationsthereof; and the gate end dielectric layer includes a dielectricmaterial selected from Si₃N₄, a metal oxide, SiO₂, or combinationsthereof.
 15. A method, comprising: receiving a structure of asemiconductor device, wherein the structure includes: a semiconductorsubstrate; a stack of first semiconductor layers and secondsemiconductor layers over the semiconductor substrate, wherein the firstsemiconductor layers and the second semiconductor layers have differentmaterial compositions and alternate with one another within the stack; adummy gate structure over the stack, wherein the dummy gate structurewraps around top and side surfaces of the stack; a first spacer onsidewalls of the dummy gate structure and over the stack; andsource/drain trenches next to the stack and exposing the first and thesecond semiconductor layers; removing a first portion of the firstsemiconductor layers exposed in the source/drain trenches under thefirst spacer to form first gaps; forming a second spacer in the firstgaps; epitaxially growing source/drain features in the source/draintrenches; forming an interlayer dielectric (ILD) over the source/drainfeatures; removing the dummy gate structure; after the dummy gatestructure has been removed, removing a second portion of the firstsemiconductor layers; after removing the second portion of the firstsemiconductor layers, circumferentially trimming a center portion of thesecond semiconductor layers; after trimming the center portion of thesecond semiconductor layers, forming a gate dielectric layer on thetrimmed center portion of the second semiconductor layers; and afterdepositing the gate dielectric layer, forming a gate electrode layerover the gate dielectric layer.
 16. The method of claim 15, wherein theremoving the first portion of the second semiconductor layers renders acenter portion of each of the second semiconductor layers to have afirst thickness, and an end portion of the each of the secondsemiconductor layers to have a second thickness, and the first thicknessis smaller than the second thickness.
 17. The method of claim 16,wherein the removing the first portion of the second semiconductorlayers renders a ratio of the second thickness over the first thicknessbeing within a range of 1.2 to
 2. 18. The method of claim 16, whereinthe removing the first portion of the second semiconductor layersrenders the second thickness being larger than the first thickness bymore than 0.5 nm.
 19. The method of claim 15, wherein the removing thefirst portion of the second semiconductor layers includes removing witha first solution, the first solution includes de-ionized water, ozone,and hydrofluoric acid, and a molar ratio of the de-ionized water to thehydrofluoric acid is within a range of 1:50 to 1:2000.
 20. The method ofclaim 15, wherein the removing the first portion of the secondsemiconductor layers includes removing with a second solution, and thesecond solution includes ammonia water.